Circuit interrupter construction



R. H. HARNER CIRCUIT INT March l0, 1959' ERRUPTER CONSTRUCTION Filed Sept. ll, 1957 INV ENTOR. Zobe/Z Har/zer,

United States Patent O 2,811,322 y yCIRCUIT INTERRUPTER CONSTRUCTION Robert H. Hamer, Park Ridge, Ill., assignor to S & C

Electric Company, Chicago, Ill., a corporationof Delaware Application September 11, 1957, Serial No. 683,424 11 Claims. (Cl. 200-120) This invention relates, generally, to the construction of power fuses of the type used on high voltage circuits of the order of 69 kv. and above where corona problems arise and it has particular relation to the provision of a corona shield for such devices shown, for example, in U. S. Patent No. 2,651,695, issued September 8, 1953, and in U. S. Patents Nos. 2,662,138 and 2,662,139', both issued December 8, 1953.

Fuses of the type shown in these patents employ a movable conductor in the form of a rod-like terminal in a bore in arc extinguishing material in which the arc is`-.drawn and extinguished. The conductor is held in place by fusible means which blows on the occurrence of an overload. In the unblown condition, the conductor is energized at the line voltage which, as indicated, may be of the order of thousands of volts. Under these conditions there is a tendency for corona to discharge from the conductor, particularly in fuses of low ampere rating when a conductor of relatively small diameter is used in a corresponding small bore to facilitate interruption of low currents. This results in the formation of nitrous oxides which, in the presence of moisture, form nitric acid or fumes thereof that attack the copper forming the movable conductor. After the conductor has been subjected to such action for a long period, it may become severely corroded and the proper operation of the fuse under short circuit conditions may be seriously impaired if the corrosion is so severe that movement of the conductor through the bore is impeded,

Accordingly, among the objects of this invention are: To prevent corona discharge from the movable conductor of a high voltage fuse in a new and improved manner; to provide a semi conducting path between the fuse terminals over the surface of the bore therebetween in which the conductor is located; to extend the semi-conducting path over the entire periphery of the bore and throughout its full length; to essentially remove or destroy the semi-conducting path by the heat ofthe arc and thereby develop adequate dielectric properties of the blown fuse; and to employ for the semi-conducting path a water resistant material which completely covers the surface of the bore and preventsr deterioration of the same.

Other objects of this invention will, in part, be obvious and in part appear hereinafter. f

This invention is disclosed in the embodiment thereof shown in the accompanying drawings and it comprises the features of construction, combination of elements and arrangement of parts that will be exemplified in the construction hereinafter set forth and the scope of the application of which will be indicated in the appended claims.

For a more complete understanding of the nature and scope of this invention reference can be had to the follow ing detailed description, taken together with the accompanying drawing in which: l v

. Figure 1 is a vertical sectional view of a fuse housing in which the present invention is embodied; and

Figure 2 is a vertical sectional view of a replaceable fuse assembly employing the housing shown in Figure 1. Referring now particularly to Figure 1 of the drawing it will be observed that the reference character 10 designates, generally, a fuse housing which forms a part ofy a replaceable fuse assembly that is indicated, generally, at 11 in Figure 2. The fuse assembly 11 is of the type shown in U. S. Patent No. 2,651,695 and can be employed in connection with the fittings there shown to prov vide a dropout fuse construction. The present inventionA can be employed with other fuse constructions, for example, the constructions shown in U. S. Patents Nos. 2,662,138 and 2,662,139 as well as other similar fuses that are used on high voltage circuits.

The fuse housing 10 includes a fuse tube 12 which,

preferably is formed of a phenolic condensation product'and may be provided with a fiber liner 13 that is'y threaded therein as shown in U. S. Patent No. 2,662,138. Within the ber liner 13 and intermediate the ends of throat 16 isa bore 17 from the surface of which an arc` extinguishing medium is evolved due to the heat of an.

arc that is drawn on operation of the fuse. Movably mounted in the bore 17 is a conductor 18 that preferably is in the form of a rod-like terminal which is provided with a contact member 19 at its upper end for connection to one terminal that is indicated, generally, at 20. The terminal 20 includes contact fingers 21 which engage directly the contact member 19 and which are suitably connected to a conducting tube 22 that has a close iit with the upper end of the fuse tube 12 and is connected to anupper terminal ring 23. It will be understood that the upper terminal ring 23 is detachably connected toa tting which forms a part of the dropout fuse assembly. In order to move the conductor 18 through the bore 17 a coil compression spring 24 is employed. It is inter, posed between the lower end of the conducting tube 22 sion 26 from the upper end of the conductor 18.

At its lower end the conductor 18 is restrained against' upward movement under the inuence of the coil compression spring 24 by a fusible means indicated, generally, at 28. The fusible means 28 may include a strain wire 29 and a fuse element 30 in parallel relation therewith. The fusible means 28 interconnects the lower arcing end of the conductor 18 with a terminal shown, generally, at 31 at the lower end of the fuse tube 12. The terminal 31 includes a relatively thick-walled metallic tube 32 which bears against the lower end of the throat 16 and to which the fusible means 28 is connected by a terminal fitting 33. Extending downwardly from the tube 32 is a conducting tube 34 which is connected at its lower end to the lower end of a lower ferrule 35 by means of rivets 36.

It will be understood that the conductor 18 in the bore 17 is energized at the potential of the line to which the fuse assembly 11 is connected. When this potential is highwith respect to the diameter of the conductor 18, a corona discharge is likely to take place from the conductor 18. As indicated hereinbefore this Acauses an undesirable condition tending to corrode the conductor 18 and render the device unsuitable for proper operation to interrupt fault current.

In accordance with this invention provision is made for preventing the corona discharge from the conductor 18. For this purpose, as shown in Figure l, after the cakes 14 of arc extinguishing material, the reinforcing ring 15 and the throat 16 have been assembled in place as there illustrated, a semiconducting water resistant coating 40 is Patented Mar. 10, 1959 applied over the entire periphery of the bore 17 andY throughout its entire length extending through the reinforcing ring 15 and throat 16. The coating 40 also extends to cover the inner surface of the fuse tube 12 at the upper end for some distance beyond the reinforcing ring 15. At the lower end the coating 40 extends for some distance over the exposed portion of the liber liner 13 and below the throat 16. On the subsequent completion of the assembly of the fuse, as shown in Figure 2, the conducting tube 22 at the upper end and conducting tubes 32 and 34 at the lower end make good contact engagement with the coating itl. ln the completed assembly, the semi-conducting coating at) interconnects the terminals 20 and 31 and provides a semi-conducting path therebetween over the surface of the bore 17 Since there is a conducting path completely surrounding the movable conductor 18 in the bore 17, it is fully shielded and no corona discharge takes place therefrom.

The semi-conducting coating 40 preferably is a colloidal graphite dispersion in a suitable water insoluble carrier, such as an alkyd resin. The properties of the coating solution and the nature of the surface of the bore 17 should be such that the semi-conducting coating 40 does not appreciably penetrate this surface of the cakes 14 of boric acid. That is, the porosity and hardness of the bore surface, and the consistency, particle size and carrier of the coating solution should be compatible. The reason for these characteristics is to make the semi-conducting coating itl readily available to the arc heat for decomposition and removal thereby, thus permitting recovery of the dielectric properties of the fuse after the arc is extinguished. The resistance of the semi-conducting coating 4l) is controlled by the graphite content which can be varied by the use of a suitable aromatic solvent. The semi-conducting coating 4d can be applied to the subassembly, as show-n in Figure 1, by pouring theV liquid solution through the fuse housing 1i) in a sufficient quantity to insure that all of the surfaces mentioned are thoroughly covered. Also, it can be applied by spraying or brushing. Preferably the resistance of the semi-conducting coating 40 between the terminals Z0 and 31 is of the order of from 2 to 20 megohms but this range is not critical.

Since the semi-conducting coating 40 is applied over the surface of the bore 17, it is largely removed on the drawing of an arc when the fusible means 28 blows. Thus, after it has served its purpose of preventing lthe formation of corona, the semi-conducting coating 40 is destroyed and, as a result, adequate resistance and dielectric strength between the terminals 20 and 31 can be developed with no danger of excessive leakage current flow therebetween due to the presence of a low resistance conducting path.

Not only does the semi-conducting coating 40 prevent the formation of corona but also it serves to prevent the deterioration of the surface of the bore 17. Since the semi-conducting coating 4d, as described, is water resistant, it prevents damage to the surface of the bore under unusual conditions of moisture accumulation which might result from high humidity and temperature cycling causing condensation or the remote possibility of water leakage into the fuse assembly under particularly severe weather conditions such as a driving rain.

Because the semi-conducting coating (it) over the surface of the bore 17 is removed or destroyed when the arc is formed on blowing of the fusible means 28, there is substantially no interference with the arc interrupting and dielectric properties of the fuse assembly 11. This follows from the fact that the rapid disintegration or removal of the semi-conducting coating 40 exposes the surface of the bore 17 in the cakes 14 of arc extinguishing material directly to the heat of the arc and the arc extinguishing medium can be evolved therefrom substantially without interference. Since the semi-conducting path has been removed from the greater portion of the distance between the terminals 20 and 31, the dielectric properties of the fuse assembly 11, after blowing of the fusible means 28, approach those of a fuse in which no semi-conducting coating has been used.

In the past, semi-conducting coatings for corona prevention have been applied to the external surface of the fuse tube, such as the external surface of the tubc 12. Also, they have been applied along the joint between the fiber liner, such as the liner 13, and the inner surface of the insulating fuse tube, such as the tube 12. However, when such use is made of semi-conducting paths between the terminals of the fuse assembly and since they remain after the fuse operates to clear the circuit, they interfere with the dielectric structure such that the liashover voltage is reduced during and following operation. Also, there is a possibility that excessive leakage current may flow between the terminals of such a fuse construction and tracking may occur following a fault interruption.

When the semi-conducting coating 40 is applied, as described hereinbefore, the fuse assembly 11 is not subject to the foregoing disadvantages. During its steady state operation, the semi-conducting coating 40 serves as an effective corona shield because of its relatively close proximity to the conductor 18. its resistance can be considerably less than that of semi-conducting coatings applied over the surface of the fuse tube or between it and the liber liner. The reason for this is that, when the v semi-conducting coating 40 is applied over the surface of the bore 17 and is destroyed whenthe fuse blows, `"it does not affect the dielectric properties of the fuse 1assembly 11 during or following an operation to the point of preventing successful clearing of the circuit, while, when thesenii-conducting coating is applied otherwise, as described, its inuence on the Vdielectric properties during and following the fuse operation is inversely related to the resistance of such a permanent coating. Since the effectiveness of a corona shield, when the fuse is energized to carry current, is inversely related to its resistance, it is desirable to employ a coating having a minimum of resistance. However, a low resistance corona shield is objectionable from a dielectric standpoint after the fuse has operated to clear the circuit unless it is substantially removed in the fuse operation. As described hereinbefore, when the semi-conducting coating 40 is applied over the surface of the bore 17, it is removed as a result of the heat of the arc and consequently any objection to the use of a relatively low resistance corona shield is overcome.

Actual tests on a device constructed as described hereinbefore show the effectiveness of the semi-conducting coating 40 over the surface of the bore 17. Accelerated corona tests up to 2.00 kv. made on a fuse assembly intended for operation on a 69 kv. circuit and employing for the conductor 18 a bare copper rod having a diameter of one-eight (1/8) inch showed that there was no evidence of corrosion on the copper surface. Comparable tests on a similar fuse assembly but omitting the semiconducting coating 40 indicated the presence of some nitrates at as low a voltage as 30 kv.

Further tests were made to determine whether the semi-conducting coating 40 had any influence on the arc extinguishing characteristics of the cakes 14 of arc extinguishing material. Currents ranging from extremely low to moderately high values at various voltage levels were interrupted. lt was found that the arcing time and over-all fuse performance were not significantly affected by the prior presence of the semi-conducting coating 40. In every test, the coating material 4h over the surface of the bore 17 was removed to such an extent that its dielec1 tric properties were satisfactorily restored. This was borne out by dielectric tests made on blown fuses, some shielded with Vthe semi-conducting coating 40 as described herein and others without it. These tests showed that the leakage current values for fuses energized at normal voltages were essentially the same for both the shielded asvaaaa and the unsbielded fuses. Breakdown voltages for the shielded fuses were essentially the same as for Unshielded" fuses and were adequate in every case to withstand voltages appearing across the fuse following arc inten ruption.

Since certain further changes can be made in the foregoing construction and different embodiments of the invention can be made without departing from the spirit and scope thereof, it is intended that all matter shown in the accompanying drawing` and described hereinbefore shall be interrupted as illustrative and not in a limiting sense.

What is claimed as new is:

1. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a fuse tube of insulating material having a bore intermediate its ends in which an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and means providing a semi-conducting path between said terminals over the surface of said bore to electrostatically shield said movable conductor and prevent corona discharge from the surface thereof.

` 2. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a fuse tube of insulating material having a bore intermediate its ends in which an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and means providing a semi-conducting path between said terminals over the surface of said bore to electrostatically shield said movable conductor and prevent corona discharge from the surface thereof, said semi-conducting path being formed by a coating of semi-conducting material overlying substantially the entire periphery of the bore surface.

3. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a fuse tube of insulating material having a bore intermediate its ends in which an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and means providing a semi-conducting path between said terminals over the surface of said bore to electrostatically shield said movable conductor and prevent corona discharge from the surface thereof, said semi-conducting path being characterized by being essentially destroyed along that portion thereof that is subjected to the heat of the arc.

4. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a fuse tube of insulating material including a filling of boric acid intermediate its ends having a bore therethrough in which an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and means providing a semi-conducting path between said terminals over,

the surface of said bore to electrostatically shield said movable conductor and prevent corona discharge from the surface thereof, said conductingv` path being provided by a semi-conducting coating comprising conducting material dispersed in a water insoluble carrier.

5. In a fuse for use on electric power circuits energzed at several thousand volts, in combination, a fuse tube of insulating material including a filling of boric acid intermediate its ends having a bore therethrough in 'fwhich an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and means providing a semi-conducting path between said terminals over the surface of said bore to electrostatically shield said movable conductor and prevent corona discharge from the surface thereof, said conducting path being provided by a semi-conducting coating comprising colloidal graphite dispersed in an alkyd resin carrier.

6. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a fuse tube of insulating material including a filling of boric acid intermediate its ends having a bore therethrough in which an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and means providing a semi-conducting path between said terminals over the surface of said bore to electrostatically shield said movable conductor and prevent corona discharge from the surface thereof, said conducting path being provided by a semi-conducting coating, the surface of said bore and said semi-conducting coating being characterized by the former being substantially unpenetrable by the latter.

7. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a fuse tube of insulating material having a bore intermediate its ends in which an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and means providing a semi-conducting path between said terminals ...over the surface of said bore to electrostatically shield "said movable conductor and prevent corona discharge f from the surface thereof, said semi-conducting path being formed by a coating of semi-conducting material overlying substantially the entire periphery of the bore surface and being water resistant to prevent deterioration of said bore surface due to moisture in the atmosphere.

8. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a fuse tube of insulating material having a bore intermediate its ends in which an arc can be drawn and extnguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and means providing a semi-conducting path between said conductor in said bore connected terminals over the surface oftsaidbore to electrostatically shield said movable conductor and prevent corona discharge from the surface thereof, said semi-conducting path being formed by a coating of semi-conducting material overlying substantially the entire periphery of the bore surface and being water resistant to prevent de terioration of said bore surface due to moisture in the atmosphere, said conducting path being further characterized by being essentially destroyed along that portion thereof that is subjected to the heat of the arc.

9. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a 'fuse tube of insulating material having a bore intermediate its ends in which an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, said conductor being movable through said bore on blowing of said fusible means, and a water resistant coating over the surface of said bore to prevent deterioration thereof due to moisture in the atmosphere, said coating being further characterized by being readily removed by the heat of the arc to permit the arc extinguishing medium to be evolved from said bore surface.

l0. In a fuse for use on electric power circuits energized at several thousand volts, in combination, a fuse tube Vof insulating material having a bore intermediate its ends in which an arc can be drawn and extinguished due tothe evolution of an larc lextinguishing medium therefrom resulting fromrthe heat of the arc, and a water resistant coating over the surface of said bore to prevent deterioration thereof due to moisture in the atmosphere, said coating being further characterized by being readily removed along that portion thereof that is subjected to the heat of the arc to permit the are extinguishing medium to be evolved from said bore surface.

11. n a fuse for use on electric power circuits ener gized at several thousand volts, in combination, a fuse tube of insulating material having a bore intermediate its ends in which an arc can be drawn and extinguished due to the evolution of an arc extinguishing medium therefrom resulting from the heat of the arc, stationary terminals at the ends of said bore, a conductor in said bore connected at one end to one end terminal, fusible means interconnecting the other end of said conductor to the other end terminal, saidr conductor being movable through said bore on blowing of said fusible means, and a water resistant semi-conducting path surrounding said conductor and connected to said terminals to shield said conductor electrostatically.

References Cited in the le of this patent UNITED STATES PATENTS 1,730,716 Austin Oct. S, 1929 1,821,761 Lemmon Sept. 1, 1931 2,048,464 Pittman et al `luly 21, 1936 2,115,436 Utsunomiya Apr. 26, 1938 2,593,426 Fahnoe Apr. 22, 1952 

